If we screw up the Earth’s biosphere, can we move to another planet?
We might just possibly be able to put a few humans on Mars. This would probably only be on a one-way ticket, though – the return trip, if possible at all, would almost certainly only be to Mars orbit and back to Earth, not to the Martian surface and back. No way can humanity emigrate there en masse, much less to any more distant planet.
Nor can we “terraform” other planets – it would be far, far easier to “reterraform” an overheated Earth.
Not at all easy, of course – it would probably take hundreds of years if not thousands, and only support a tiny fraction of our current population in the meantime. As for decontaminating a radioactive Earth – that’s not possible at all, although time will heal it eventually.
(This is little more than a placeholder for a bigger piece.)